Please feel free to drop your own ideas on how to increase profit in businesses.
customer is king and should be treated like a royalty.
Not always. There are some "toxic" customers and time-wasters who come to your shop without intention to buy anything. You will lose your time and efforts to sell but the deal is impossible as they were not even considering buying anything initially.
Stock different brands of the products that you sell.
Sometimes this works against you too. If you sell items of one or few specific brands, you have your own loyal customers, easier to control stock, do accounting, buy items etc. This saves a vast amount of time and work. The more time and energy you have, the more you can achieve.
You're correct about toxic customers who just walk into a shop without any intention of buying anything. I personally treat all my customers with utmost respect, because there's no way of knowing a serious customer unless when you're attending to them. But if I'm convinced that they've come to make trouble, I'll politely ask such a person to leave my shop. Stocking specific brands works for big business people, but small business people like me sells more when we stock different varieties of brands that customers can choose from.